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Thursday, 1 May 2014

Dolby System

The Dolby System is extensively used for reduction of noise
in audio video systems. There are three Dolby methods.

Dolby A, Dolby B, Dolby C.

Dolby A: Used for professional recording, which is a complex
method.

Dolby B: Used for domestic recording and is comparatively
simpler method.

Dolby C: This is the recent developed method and is very
effective in the noise reduction.

The basic dolby system (Dolby B) – used for domestic
recording:

In this system, the signals below a pre-determined level
called ‘dolby level’ (say below 40 db and above 1 KHz) are emphasized (boosted)
while recording (coding) and are de-emphasized (reduced) back during replay (decoding)
in the same amount.

As the high frequencies have low intensity, therefore they
are passed through a high pass filter circuit. They are then amplified and sent
to a limiter circuit, which limits them to a constant amplitude. Now the
limited signal and the splitted signals are added by an added circuit.

The reverse is done in the replay process. The Figure (a)
shows circuit for recording (coding) and Figure (b) for replay (decoding).

Figure (a)

Figure (b)

Note:

1.The noise of the tape recorder is basically a hissing
noise during recording a low level audio signal.